Not clicking "received"

wait 7 days and if notthing happen
https://www.steamgifts.com/support/tickets/new

request recieved feedback

or

request new winner

You can easily avoid this by making GA with some level restriction.

You can see if they saw the key on the GAs page. If there is an eye next to it they saw it, if it isn't there they still haven't seen the key.
Sometimes I waited for a few days just for people to see the key even if they are online on SG so be patient :)

First:
Welcome at steamgifts :o)

Second:
The most that take long to set the recieved key are low level people that have a lot of bad entries (Red marks) at sgtools.info
They are each day a lot online but only react after 4 days+.
Looks as they use autojoiners..... (and there winning behavior speaks for that too).

You can only WAIT 7 days
Try to reroll (request new winner) for all that not click the recieved button after 7 days.

A good idea will also be to read the FAQ. It covers most things nicely.

Well one of your giveaways ended 6 hours ago. So it's quite short for people activating it.

And some people, well, are pretty damn slow to respond, or mark as received.

Low level public users are unreliable, be it in adhering to site rules, in accepting won games, or anything else.
If reliability is your aim, you'll want to avoid the bot- and rulebreaker-heavy levels 0, 1, and 2, and consider posting around level 4 as a minimum. For games you care about (or when you just generally want consistent reliability of winners), level 6+ or forum giveaways are where you tend to see more reliability in games "going to a good home", though SGT filtered invite giveaways work better, and specialized groups work the best for such aims.

You can check your winner's profile against SGTools (https://www.sgtools.info/activation and https://www.sgtools.info/multiple-wins) to see if they have any outstanding violations, which (by mentioning the existence of the infraction through a Request New Winner ticket) can potentially get you an immediate reroll on the winner, regardless of how much time has passed since the giveaway ended. Otherwise, to make sure the winnner has had adequate time to respond, you have to wait 7 days before you're able to Request New Winner (if the key hasn't been viewed) or Request Received Feedback (if the key has been viewed).

Of course, everyone has their own circumstances, and I've had a few winners now that have taken weeks or months to claim their win (Mind, rules would have permitted me to reroll any time after 7 days had passed, had I wished to). [More typically, however, users seem to tend to mark received at some point between 8 hours and 3 days after I send the key.] As such, don't assume that every delay is done out of disrespect or site exploitation.

Do keep in mind that while lower levels and public entrants will far more reliably ignore your giveaway descriptions (in large part because they're typically entering through methods where they can't see the descriptions to begin with), it's generally best to specify any preferences you have (ie, "Please activate and mark received quickly" or "Please don't spam me with generic messages") within your giveaway descriptions. It doesn't always help, given how many users ignore such things, but any user of the site that is willing to treat others with respect will give such preferences proper consideration, if the request is phrased respectfully.

Finally, as a new user, you could consider specialized groups like Unlucky 7, which can significantly improve your win rate or the speed at which people mark received on your giveaways. Do note that private and group giveaways do require a minimum of five entries to give you CV (account experience), so you'll want to be careful about the duration and appeal of the game you put up for such giveaways, relative to the number of users within the group.

Given time and experimentation, you'll be able to work out what time-frames and entry restrictions work best for you. :)